Just as an extension on what Only A Hobo said, closing & re-opening LW will normally do the trick.

I don't normally use the LW Playlist on iTunes so I'm not sure how well this updates.

iTunes will not import songs that are already in the library with identical characteristics; ie: same title, same size, same id3 tag. But it will import same title if the id3 tag has differences.

Be aware that iTunes will not import mp3pro files. You can import Ogg Vorbis files "if" you acquire a special plug in. See the Sticky at the top of this section which has a posting about iTunes & iPod. There's some useful tips & links there.